1. Field of the Invention
This invention relates generally to thermometry, and more particularly to a temperature controlled null radiometer suitable for use as a biomedical thermometer.
2. Description of Related Art
Variations in the internal temperatures of temperature measuring instruments tend to affect the calibration of such instruments. Temperature measuring devices therefore may include a mechanism for maintaining a relatively constant internal temperature. Frequently this takes the form of a heat sink which will stabilize the internal temperature of the device, but will also permit the internal temperature of the device to slowly adjust to ambient temperature. More active systems for controlling internal temperature of a temperature measuring device have also been used. For example, it has been suggested to provide an infrared transducer-transmitter for noncontact temperature measurement with a liquid cooling jacket for maintaining the internal temperature of the device at a relatively constant temperature. A thermistor would be used in a feedback loop with an amplifier altering the gain of amplification of the temperature detector, to compensate for internal temperature fluctuation. A system for heating or cooling a thermal reference source of an infrared thermometer, based upon a measured temperature of target is also known. The temperature of the reference source is maintained at a constant temperature near the target temperature, to provide for more accurate readings.
A unique tympanic thermometer utilizing a system for heating and cooling of air blown into the ear canal is also known. The temperature of the air is measured, and controlled until a temperature equilibrium is reached between the ingoing air and the outgoing air. A null temperature difference between the ingoing and outgoing air serves as an indication of when a temperature reading of the air should be used to determine the temperature of the inner ear. Unfortunately, this system suffers from a disadvantage of introducing temperature changes into the target area. A procedure for providing an equilibrium temperature reading in a chopper stabilized null-type radiometer utilizing a pyroelectric receiver is also known. The null-type procedure is useful in overcoming an inaccuracy in the temperature coefficient of the pyroelectric receiver, which has an output current proportional to the time rate of change of a temperature difference. Another type of chopper stabilized infrared thermometer utilizes an associated calibration unit maintained at a reference temperature, which the thermometer probe compares with temperature readings of a target.
It would be useful to provide an infrared thermometer utilizing a null-type procedure in which the output of the radiation detector is proportional to the difference between the temperature of the target and the known temperature of the radiation detector, so that when a zero reading is measured, the temperature of the target can be determined, to provide a direct measurement of temperature. The present invention fulfills this need.